Charge separation, charge recombination and intersystem crossing in orthogonal naphthalimide–perylene electron donor/acceptor dyad

• Published in: Photochemical & photobiological sciences 2021, Vol.20 (1), p.69-85
• Main Authors: Chen, Xi, Pang, Junhong, Imran, Muhammad, Li, Xiaolian, Zhao, Jianzhang, Li, Mingde
• Format: Article
• Language: English
• Subjects: Biochemistry; Biomaterials; Chemistry; Chemistry and Materials Science; Original Papers; Physical Chemistry; Plant Sciences
• ISSN: 1474-905X; 1474-9092
• DOI: 10.1007/s43630-020-00002-w

We prepared an orthogonal electron donor/acceptor dyad ( NI–Py ) with perylene (Py) as electron donor and 4-aminonaphthalimide ( NI ) as an electron acceptor. The molecule adopts orthogonal geometry due to the steric hindrance exerted by the 4-amino substituents on the NI moiety. The photophysical properties of dyad were studied by steady-state UV–Vis absorption and fluorescence spectroscopies, femtosecond/nanosecond transient absorption spectroscopies and DFT computations. Ground state interaction between the NI and Py units is negligible; however, charge separation occurs upon photoexcitation, indicated by the quenching of the fluorescence of the dyad in polar solvents, i.e. fluorescence quantum yield ( Φ F ) is 61.9% in toluene and Φ F  = 0.2% in methanol. Spin–orbit-coupled charge transfer-induced intersystem crossing (SOCT-ISC) was confirmed by femtosecond transient absorption spectroscopy (charge separation takes 1.7 ps and charge recombination takes 6.9 ns, in CH 2 Cl 2 ). Nanosecond transient absorption spectra indicated the formation of perylene-localized triplet state, and the triplet state lifetime (175 μs) is much longer than that accessed with the heavy atom effect (3-bromoperylene; 16 μs). The singlet oxygen quantum ( Φ Δ ) yield of the dyad is 2.2% in hexane and 9.5% in dichloromethane. The low SOCT-ISC efficiency as compared to the previously reported analogue ( Φ Δ  = 80%) is attributed to the mismatch of the 1 CT/T n state energies, and/or the orientation of the NI and Py units, i.e. orthogonal geometry is not sufficient for achieving efficient SOCT-ISC in compact electron donor/acceptor dyads. Graphical abstract